Tweak base/version.jl printing a bit

* I don't like printing "Unknown commit" when there's not `git` information, just print out what we know and leave it at that

* versioninfo() depended on the `commit_string` variable, but that gets changed when on a tagged release now.  I've changed it to just build its own output internally, when we have the information available.  One less weird global variable floating around!